Thyroid peroxidase is involved in two very important steps of thyroid hormone synthesis: iodination of thyronine residues in thyroglobulin and intramolecular coupling of these iodinated tyrosines, leading to the production of triiodothyronine (T3) and thyroxine (T4). Thyroid peroxidase has been identified as one of the major autoantigens, antibody against which is found in sera of patients with autoimmune thyroid disorders such as Graves' disease and Hashimoto's thyroiditis. In order to understand the molecular mechanism of thyroid hormone synthesis and of thyroid autoimmunity, we have cloned cDNAs and the gene for human thyroid peroxidase and a cDNA for rat thyroid peroxidase. We have also studied the mechanism of regulation of thyroid peroxidase gene expression. Two approaches were taken. First, we used the rat FRTL-5 thyroid cell line as a model system to explore the effects of various hormones and growth factors on the expression of the thyroid peroxidase gene. Rat FRTL-5 is the only available continuous normal thyroid-originated cell line which maintains most of the functional thyroid characteristics. Expression of the thyroglobulin gene, another key protein in thyroid hormone synthesis, was also examined in parallel with that of the thyroid peroxidase gene. The results clearly indicated that different mechanisms are regulating thyroid peroxidase and thyroglobulin expression. A second approach to study the regulation of thyroid peroxidase gene expression was to directly examine the human thyroid peroxidase gene upstream DNA sequence for promoter and/or enhancer activities. We have identified the 230-base pair sequence around 5.5 kilobase pairs upstream of the gene transcription start site, which appears to have a thyroid specific enhancer activity.